Response of the Bohr group salt bridges to ligation of the T state of haemoglobin Kansas

Abstract

In his stereochemical mechanism for haemoglobin, Perutz (1970) proposed that ligation of the T state of haemoglobin caused tertiary structure changes culminating in the breakage of Bohr group salt bridges. The bonds changed or broken in this pathway are partially responsible for the free energy of co-operativity. The Perutz scheme was supported by kinetic and equilibrium data on haemoglobin A but not by Anderson's (1975) crystallographic work on haemoglobin Kansas, (Asn 102 (G4)β→Thr)showing the Bohr group salt bridges to be intact in the liganded T state. We have confirmed Anderson's findings by showing that the p K of a Bohr group, His 146β, remained unchanged on ligation of the T state of haemoglobin Kansas with NO, and that there was little T state Bohr effect with either NO or oxygen. To reconcile these seemingly contradictory findings in haemoglobins A and Kansas we propose that the unusual properties of the liganded T state of haemoglobin Kansas are part of a general occurrence and will be expressed in any haemoglobin with low ligand affinity whether it be caused by inositol hexaphosphate, crystallization into the deoxy form, or ligation with NO or high-spin ligand. This would mean that there would be no unique stereochemical mechanism for haemoglobin.